1. Technical Field
This invention relates to an operation control device for a vacuum pump and a method for stopping the operation of the vacuum pump. This invention relates in particular to an operation control device for a vacuum pump for use in evacuating the interior of a chamber of a semiconductor manufacturing apparatus or the like, and to a method for stopping the operation of the vacuum pump.
2. Related Art
Vacuum pumps are widely used in semiconductor manufacturing apparatuses to evacuate gas used in the semiconductor manufacturing process from the chamber and to make vacuum environment in the chamber. As for vacuum pumps, such types are known as the positive-displacement type provided with pump rotors of Roots or screw type.
Generally, the positive-displacement vacuum pump is provided with a pair of pump rotors disposed in a casing and an electric motor to drive and rotate the pump rotors. Between the paired pump rotors and between the pump rotors and the casing, very narrow clearances are formed; and the pump rotors are adapted to rotate without contacting the casing. As the paired pump rotors rotate synchronously in opposite directions, gas in the casing is moved from the suction side to the delivery side; and the gas is evacuated from the chamber or the like connected to the suction port.
Some of gasses used in the semiconductor manufacturing process contain constituents that solidify or liquefy at low temperatures. Generally, as the above-mentioned vacuum pump generates compression heat in the process of moving the gas, the vacuum pump in operation is heated up to a certain temperature. Accordingly, as long as the vacuum pump is kept at high temperatures, even when a gas containing the above constituents is evacuated using the above vacuum pump, the constituents do not solidify or liquefy, so that favorable evacuation is carried out.
[Patent Document 1]    JP-A-2004-138047
However, when the vacuum pump stops operation and its temperature lowers gradually, the constituents contained in the gas solidify or liquefy and end up in accumulating in gaps between the pump rotors and between the pump rotors and the casing (the solidified or liquefied constituents will be hereinafter called “products”). When the temperature lowers further, the pump rotors and the pump casing shrink, and gaps between them become narrower, and the products end up in being squeezed between those gaps. As a result, there have been problems as follows: The squeezed products hinder the rotation of the pump rotors, so that the pump rotors cannot be rotated with the starting torque of the electric motor, and the vacuum pump fails to restart. Moreover, under such a condition, not only the vacuum pump cannot be restarted, but also the electric motor is overheated due to overload and the vacuum pump is hindered from being operated safely.
Besides, in recent years, a motor drive technique has been in progress in which an induction motor using a frequency converter, a brushless DC motor, etc. are driven. When such a motor drive technique is used in the vacuum pump, the motor torque for starting the vacuum pump is finally determined with the capacity of components used in the frequency converter. As a result, the condition for starting the vacuum pump is becoming severer because the electric motor cannot produce torque greater than a certain limit.
This invention has been made in view of the above point. Therefore, the object of this invention is to provide an operation control device for a vacuum pump and a method for stopping the operation of the vacuum pump, making it possible to effectively remove the products when the vacuum pump is going to be stopped and normally start the vacuum pump even when the solidified or liquefied products in the casing may otherwise hinder the rotation of the pump rotors.